FIG. 10 is a perspective view of a conventional, biaxially driven optical reflecting element. Optical reflecting element 1 includes a pair of vibrators 4 and 8, frame 5, a pair of vibrators 6 and 7, and mirror 9. Vibrators 4 and 8 are connected to the end of mirror 9. Frame 5 is connected to vibrators 4 and 8, and encloses the outer circumference of vibrators 4 and 8 and mirror 9. Vibrators 6 and 7 are connected to frame 5 at each end outside frame 5. Vibrators 4 and 8 have central axis S11 parallel with the Y axis; vibrators 6 and 7 have central axis S12 parallel with the X axis. Vibrators 4, 8, 6, and 7 are meander-shaped.
Each of vibrator 4, 8, 6, and 7 has a drive element (not shown) disposed thereon composed of a bottom electrode layer, a piezoelectric layer, and a top electrode layer. Applying voltage to this drive element enables rotating mirror 9 about central axes S11 and S12. Launching light to mirror 9 allows its reflected light to scan on the flat surface of a screen, thereby projecting images on such as a wall and screen.
Each of vibrator 4, 8, 6, and 7 and mirror 9 are further provided thereon with a monitor element (not shown) composed of a bottom electrode layer, a piezoelectric layer, and a top electrode layer. When an electric signal detected with the monitor element is input into the upper electrode of the drive element through a feedback circuit, optical reflecting element 1 can be always driven at a resonance frequency theoretically. Such a self-excited driving method allows maintaining a large amplitude. Such optical reflecting element 1 is disclosed in patent literature 1 for example.
In recent years, highly accurate self-excited driving has been required in biaxial optical reflecting elements. This is because highly accurate self-excited driving stably produces a large deflection angle independently of variation in resonance frequency. In a biaxial optical reflecting element, however, an electrode for monitoring the X axis is structurally forced to pass over a beam for driving the Y axis. Consequently, the electrode for monitoring the X axis results in receiving a signal in a higher-order vibration mode from the Y-axis-driving beam. The signal becomes spurious resonance noise, which prevents appropriate self-excited driving.